Electrochemical cells may be classified as primary or secondary. Primary cells are those that derive electrical energy from a chemical state and are generally not rechargeable. Secondary cells are rechargeable electrically by passing a current through the cell in a direction reversed from that of discharge.
Basically an electrochemical cell is made up of two half-cells, each comprising an electronic conducting phase or electrode in contact with a second phase called an electrolyte in which ionic conduction takes place. During discharge the electrolyte loses electrons to one of the electrodes thereby oxidizing that electrode. At the other electrode the electrolyte gains electrons and is thereby reduced. The electrolyte associated with the cathode is referred to as the catholyte and that with the anode as the anolyte. In some cells the catholyte and anolyte are different solutions and therefore require a separator membrane to prevent the two solutions from physically mixing. In other types of cells the catholyte and anolyte are the same in which case the separator functions to physically separate the electrodes. These membranes should not, however, prevent ionic conductance between the catholyte and anolyte.
Examples of primary battery systems are those having as electrodes: mercury zinc; silver-zinc; lead-zinc; copper-zinc; copper-magnesium; and silver-magnesium.
Examples of the most common secondary battery systems are: nickel-cadium; silver-zinc and silver-cadium. A common electrolyte used in both the primary and secondary cells is a 30 to 45% solution of KOH. Other types of electrochemical cells in which the membranes of the present invention may be used are those used for electrolysis and dialysis.
The separator membranes used in the above system must possess certain physical and chemical properties, such as low electrolytic resistance, and high resistance to oxidation particularly in alkaline solutions at high temperatures. Furthermore, the membrane must have sufficient mechanical strength to withstand the rigors of battery assembly and also prevent dendrite growth or treeing between the two electrodes. These dendrites, if not stopped by a separator, bridge the gap between the electrodes thereby short circuiting the cell.
In the past a variety of separators have been used to prolong the life of cells. For example micro-porous materials have been tried, however, it was found that these did not prolong cell life due to the fact that they had too open a structure. Separator membranes composed of cellulosics such as cellophane have also been tried. Although these membranes have low elecolytic resistance and the capability of slowing the migration of silver oxides toward the cathode they undergo severe oxidative degradation and hydrolytic attack both of which limit the life of the cell. More recently membranes with improved characteristics have been developed using irradiation grafting techniques. Membranes of this type are disclosed in U.S. Pat. Nos. 3,427,206 and 4,012,303.
It is an object of this invention to provide improved permselective membranes for use in electrochemical cells, which have low electrolytic resistance.
It is a further object to provide a membrane which is resistant to oxidative degradation in electrolytic solutions, particularly at high temperatures.
It is a still further object of this invention to provide a novel method for the preparation of membranes and the use of such membranes in electrochemical cells.
Still other objects and advantages of the present invention will be obvious and be apparent to those skilled in the art from the specification and the appended claims.